This invention relates to processes for increasing the dry strength of paper using mixtures of anionic dry strength resin and cationic or amphoteric starch.
In paper-making processes, starch is frequently added to the pulp fiber slurry prior to the sheet forming operation to increase the final strength, particularly dry strength of the paper or paperboard. The starch can be added as an unmodified, uncooked slurry. However, addition in this form usually results in poor retention of the starch in the paper, incomplete swelling of the starch granules and concomitant low strength development. Therefore, most starch added to paper-making pulp slurries is cationic starch, which is generally made cationic by reaction of the starch with a cationic modifier, such as 2,3-epoxypropyltrimethylammonium chloride. The cationically modified starch is cooked to put it into solution and delivered as a high solids (15-30%) solution to the paper mill. In other cases the cationically modified starch is delivered as a dry powder and cooked at the paper mill site to make a low solids (0.5-4.0%) solution, which is then added to the pulp slurry. The cationic starch molecules can adhere to the naturally anionic pulp fibers by electrostatic attraction and thus be retained in the wet fiber mat and remain in the final paper.
There are two major problems with cationically modified starches as additives to paper-making pulp slurries. The first is that the cationic starch molecules can overwhelm the anionic charge on the cellulose fibers, thus setting a limit on the amount of cationic starch which can be added to the slurry. If too much cationic starch is added and the pulp fibers became cationic, only a portion of the starch which is added will be retained in the sheet. The rest will circulate in the paper machine white water system and cause problems such as foam, deposits on the paper-making machinery and filling of the felts that are used to press the water out of the wet fiber mat. In addition, there will be no additional strength benefit in the finished paper from the extra starch which is added, because it is not retained in the sheet.
A second problems is that fibers which have been made cationic by excessive cationic starch addition, will not be able to adsorb other cationic additives which are commonly added the to the pulp slurry, such as sizing agents, cationic dyes, cationic pitch and stickies fixatives, wet strength resins, synthetic dry strength resins and cationic retention and drainage aids. Because of this problem, there is an upper limit on the amount of starch which can be added. This limit depends on the level of anionicity of the fibers, the cationic charge density of the starch, and the amount and type of other cationic additives. This limit is typically between about 0.5 wt. % and 2.5 wt. % starch on a dry basis, based on the weight of the dry pulp fiber. In order to overcome this problem, starch manufacturers have introduced amphoteric (containing both cationic and anionic functional groups) starches. It has been found that in order for the amphoteric starch to be effectively retained it must still contain a net cationic charge. Thus the upper limit on the amount of amphoteric starch which can be added before encountering the problems enumerated above is higher than for straight cationic starches, but a limit still exists, because of the net cationic charge on the amphoteric starches. In addition, amphoteric starches are more difficult to manufacture and more expensive than cationic starches.
Another class of paper strength additives which can be added to the pulp fiber slurry is anionic polyacrylamide resins. The most commercially successful are reaction products of acrylic acid with acrylamide to from a linear acrylamide chain with anionic carboxylic acid side groups. These products can also be cross linked by reaction with a cross linking agent such as, for example, methylene bisacrylamide. These anionic dry strength agents are very poorly retained in the fiber slurry unless a separate addition of a highly cationic additive is also made to the pulp slurry. Typical cationic additives are, for example, polydiallyldimethylammonium chloride, dimethylamine/epichlorohydrin resins, epichlorohydrin/polyamidoamine resins, and polyethyleneimine. Aluminum compounds such as alum or polyaluminum chloride can also be used to retain anionic dry strength additives, but are normally only useful if the paper-making pH is lower than 6.0 and preferably lower than 4.5. Cationic starch has also been tried as a retention aid for anionic dry strength agents, but has not proven very effective.
It is generally believed that in order to effectively retain an anionic dry strength agent, the cationic fixative must have a high cationic charge density, so as to put a localized spot of very high cationic charge on the surfaces of the fibers as an attachment site for the anionic dry strength additive. Cationic or net cationic amphoteric starches typically have cationic charge densities of between 0.10 and 0.5 milliequivalents per gram dry basis and typically 0.20 milliequivalents per gram dry basis. This is much lower than the charge density of synthetic cationic fixatives which have charge densities of 1.8 to 20.0 milliequivalents per gram dry basis and typically 6.0 milliequivalents per gram dry basis. Aluminum compounds also display high cationic charge densities under appropriate acid condition, although this is difficult to measures because of the instability of these compounds in water. It is common practice to add the cationic fixative to the fiber slurry first, such as in the machine chest (last high consistency chest before the paper slurry is diluted with paper-making white water dilution), or to the pump feeding the slurry from the machine chest or to the stuff box (pressure leveling box) prior to the high consistency stock slurry being diluted with paper machine white water. The anionic dry strength agent is then added to the stock slurry at a later point in the system, separated form the cationic fixative by a point of mixing such as the machine chest stock pump, the primary cleaner pump or the fan pump. Additional points for the anionic dry strength resin include all the addition points mentioned above and in a addition a point ahead of the basis weight control valve, the suction of the primary cleaner pump, the suction of the fan pump and points before or after the primary machine screen.
There are several problems that can arise involving the use of anionic dry strength agent and cationic fixative. First, the ratio of the two must be carefully controlled. If an excess of anionic dry strength agent is added, it is poorly retained and the excess builds up in the white water loop of the paper machine causing foam and deposits. If too much cationic fixative is added, it can over cationize the fibers and hurt retention of all additives. Second, changes in the quality of the pulp slurry, the pH of the paper machine system, the ratios of various types of paper-making fibers, will all affect the optimum ratio of the cationic fixative to anionic dry strength resin. Third, the determination of the proper ratio of fixative to anionic dry strength resin must be made using sophisticated test equipment such as a streaming current detector or zeta potential meter, both of which are sensitive laboratory instruments, subject to operator error and not entirely suitable for control of an industrial processes. Fourth, the addition point of the anionic dry strength resin and the cationic fixative must be carefully selected so as not to interfere with or be interfered by the addition of other cationic and anionic additives in the stock slurry stream. In addition if the cationic fixative is added close to he addition point of inorganic filler added to the stock slurry the filler particles will adsorb the fixative and tie it up.
This invention is a process for making paper with improved dry strength comprising: a) adding to an aqueous paper pulp slurry a premixed aqueous mixture of anionic dry strength resin and cationic starch or amphoteric starch having a net cationic charge, wherein the ratio of the dry strength resin to cationic or amphoteric starch is such that the aqueous mixture has a net cationic charge, and b) sheeting and drying the treated pulp slurry to form paper. Paper produced by the process may have a higher dry strength than paper that is the same except that the anionic dry strength resin and cationic, or amphoteric starch are added separately.